Pace's strong finish on the demanding layout in Springfield, New Jersey, put her one in front of Chinese duo Lin Xiyu and Yin Ruoning and Canadian Brooke Henderson, who shared second place a stroke ahead of Thailand's Wichanee Meechai.
"I played very well today," said Pace, who claimed her lone LPGA tour title in 2014.
"Obviously I'm very happy. No bogeys, which is awesome. Putted really well, very happy with the round," added Pace, whose best finish in a major was a tie for sixth in the Evian Championship in 2015.
Pace was concentrating this season on the Ladies European Tour - where she's an 11-time winner - before heading stateside for the ShopRite Classic earlier this month.
She took a conservative approach and it paid off on a course where only 16 players broke par.
"We kept it pretty simple," she said. "Most of the greens we went for the middle, didn't really attack too many pins. The putter worked, which was awesome. Hit my driver pretty straight, as well."
Darkness was closing in as Pace warmed up. She had back-to-back birdies at 11 and 12, added another at 15 and finally birdied 18.
"At my age, the darkness isn't your friend. But, like I said, I felt very comfortable on the greens. I made a really good putt on 15, massive slopey, very quick putt. That was a huge bonus."
She got up and down from a bunker at 16 then reached the green at the par-five 18th in two and for another birdie.
Henderson, who won the Women’s PGA Championship in 2016, teed off on 10 and had two birdies and a bogey in her first nine holes before nabbing three on her inward run.
That included a birdie at her final hole, the ninth, which pulled the two-time major champion level with Lin and Yin.
"Everything was working pretty well. I would like to hit a few more fairways tomorrow, but when I needed to I was able to get up-and-down a few times and the putter was rolling," said Henderson - who hit just five of 14 fairways.
"It's nice to make five birdies out on this golf course. Hopefully just do something similar over the next few days."
Lin had two birdies on each nine without a bogey as she aims to improve on her best prior major finish, a tie for seventh at the 2021 US Women's Open.
Yin had two birdies and an eagle without a bogey, hitting 13 fairways and 17 greens in regulation.
"My ball-striking was right on spot," she said.
Yin teed off on 10 and after a birdie at 17 eagled 18, where she "just smashed my three-wood up there and ended up pretty good."
She drained a six-foot putt to make the turn three-under and added one more birdie coming in.
